In order to process a sheet for reuse, an image to be decolored is initially formed on the sheet using a decolorable color material, and then the image is decolored from the sheet by performing a decoloring process with respect to the image. For the decolorable color material, a decolorable toner may be used, for example. The decoloring toner is melted at a fixing temperature, and is fixed onto a sheet so that a color is developed. In addition, when a decoloring temperature higher than the fixing temperature is applied, an image of the decoloring toner which is already fixed is decolored.
In a decoloring unit which performs a decoloring process, only a sheet on which an image printed using the decoloring toner is fed. However, when a sheet on which an image which is printed using non-decolorable toner (hereinafter, referred to as ordinary toner) is fed in the decoloring unit by mistake, there is a problem in that an offset phenomenon occurs in which the ordinary toner is melted by being overheated. The melted ordinary toner then adheres to the face of a heating and pressuring member, such as a roller, included in the decoloring unit.
Accordingly, it is desirable to determine whether or not an image printed on a sheet is suitable for a decoloring process before performing the decoloring process in the decoloring unit.